1. Field of the Invention
The present invention relates to an optical amplifier, such as a doped fiber amplifier, using bidirectional pumping to amplify a signal light propagating through a doped optical fiber. More particularly, the present invention relates the relationship between the wavelengths of the pump light used for bidirectional pumping, and to the control of the pump light to adjust tilt gain of the doped fiber amplifier.
2. Description of the Related Art
There is increasing demand for communication systems, such as multimedia networks, which can provide high speed transmission and large bandwidth. Moreover, optical communication systems which use wavelength division multiplexing (WDM) promise to provide such high speed transmission and large bandwidth.
In optical communication systems which use WDM, a rare earth element doped fiber amplifier is typically used to amplify optical signals since this type of doped fiber amplifier has a relatively wide gain band. A wide gain band is generally required to amplify all the channels which may be included in a wavelength division multiplexed signal light.
In such a rare earth element doped fiber amplifier, either forward pumping or backward pumping can be used to amplify signal light. With forward pumping, a light source produces pump light which propagates through the fiber in the same direction as the signal light propagates. The forward travelling pump light interacts with the rare earth element doped fiber to amplify the signal light. With backward pumping, a light source produces pump light which propagates through the fiber in the opposite direction as the signal light propagates. The backward travelling pump light interacts with the rare earth element doped fiber to amplify the signal light. With bidirectional pumping, both forward and backward pump light are provided in the fiber, so that the signal light is amplified by both the forward and backward pump light.
Unfortunately, with bidirectional pumping, one of the light sources providing pump light will be influenced by pump light provided by the other light source. For example, a light source providing the forward pump light may be influenced by the backward pump light. Similarly, a light source providing the backward pump light may be influenced by the forward pump light. As a result, the gain of the doped fiber amplifier becomes instable.